1. Field of the Invention
The present invention relates to a gas barrier film (including a metal foil film) used for packaging materials used for foods, medicines, photosensitive materials and tobaccos for the purpose of preserving the contents by shielding various gases.
2. Description of the Related Art
In recent years, plastic films and sheets or molding-processed products thereof are mainly used for packaging materials for the purpose of preserving the contents because of transparency, light weight and economical efficiency. Performances required to a plastic film used for packaging foods, medicines and cosmetics include a barrier property to various gases, transparency, a retort treatment resistance, an impact resistance, flexibility and a heat sealing property. In particular, a high barrier property to oxygen and moisture is required for the purpose of maintaining the performances and the quality of the contents.
In general, thermoplastic films do not have a so high gas barrier property, and therefore a method in which a polyvinylidene chloride (PVDC) resin is coated to the thermoplastic films (PVDC-coated film) has so far mainly been used as means for providing it with a gas barrier property. However, a PVDC-coated film prepared by this method contains a halogen atom, so that a non-halogen base gas barrier film is required from the viewpoint of environmental protection.
Films prepared by vapor depositing inorganic compounds such as silica (silicon oxide) and alumina (aluminum oxide) on the surface of thermoplastic plastic films are used as a technique substituted for this. These films reveal a very high gas barrier property, but there has been the problem that because the gas barrier layer is formed by vapor depositing hard inorganic compounds, cracks and pin holes are produced on the gas barrier layer by bending and extending in mold-processing of the films, so that a marked reduction in the gas barrier property frequently takes place when actually used as a packaging material.
On the other hand, a film prepared by vapor depositing aluminum on the surface of a thermoplastic film of polypropylene is widely used mainly for contents requiring particularly a light shielding property, but the same problem as those of the films prepared by vapor depositing inorganic compounds such as silica and alumina frequently takes place as well on this film.
A metal foil, particularly an aluminum foil has been used for packaging materials for foods, medicines, photosensitive materials and tobaccos over a long period of time making the best use of an excellent gas barrier property thereof for moisture and various gases and an excellent light shielding property thereof. In applying an aluminum foil to packaging materials, it is used in combination with a sealant layer comprising mainly polyolefin and a layer comprising polyamide and polyester in order to provide it with protective and adhesive properties and processability in printing.
In recent years, a reduction in burned residues is required in order to reduce a load exerted on the environment in treating wastes by burning. However, if an aluminum foil layer in an aluminum foil-laminated film is reduced in a thickness, caused is the problem that cracks and pin holes are produced on the aluminum foil layer by bending and extending in mold-processing of the film and that a marked reduction in the gas barrier property frequently takes place when actually used as a packaging material, and a reduction in the thickness of the aluminum foil layer has been limited.
A method in which a nylon film is laminated on both sides of an aluminum foil in an aluminum foil-containing pouch comprising a laminated film of a thermoplastic film and an aluminum foil is proposed in Japanese Patent Application Laid-Open No. 77089/1997 as a method for improving an impact resistance and a pin hole resistance of an aluminum foil-laminated film. Proposed in Japanese Patent Application Laid-Open No. 314735/1997 is a laminated film having an excellent impact resistance and pin hole resistance comprising the first layer: an oriented nylon film, the second layer: a metal foil, the third layer: a co-extruded film having two LLDPE layers or an LDPE layer and a nylon layer interposed therebetween and the fourth layer: an LLDPE film or an LDPE film. Proposed in Japanese Patent Application Laid-Open No. 34821/1998 is a laminated film for packaging electronic parts which is excellent in a moisture proofing property, a pin hole resistance, a sticking resistance and an antistatic property, prepared by laminating an antistatic layer, a biaxially oriented polyethylene terephthalate layer, an aluminum foil layer, a biaxially oriented nylon layer and an antistatic polyolefin layer in this order from the surface, wherein a gas barrier layer comprising an aluminum vapor deposited layer, an alumina vapor deposited layer, a silica vapor deposited layer and a vinylidene chloride resin layer is laminated between the biaxially oriented polyethylene terephthalate layer and the biaxially oriented nylon layer described above. All of these methods intend to improve an impact resistance and a pin hole resistance by scheming a layer structure and require a complicated layer structure, that is, a complicated production process.